1. Field of the Invention
The present invention relates generally to monolithic filters, and particularly to single-fiber coaxial Mach-Zehnder and lattice devices and their applications.
2. Technical Background
There is an emerging need for narrowband wavelength division multiplex (WDM) couplers and filters. Important applications include broadband gain flattening filters for rare earth and Raman amplifiers. For example, such devices can be utilized in the 1550 nm window for modifying the gain spectrum of erbium fiber amplifiers. They will also be widely used in trunk lines as well as in fiber-to-subscriber architectures. These components must environmentally stable and very reliable.
Mach-Zehnder filters are known for their narrow-band wavelength capabilities. It has been proposed that filters having pass bands as narrow as 1 nm be formed by connecting two evanescent couplers with unequal fiber lengths between them. However, it is difficult to reproduce an environmentally stable device with this approach. The connecting fibers are subject to external destabilizing conditions such as temperature changes and random bending forces.
An environmentally stable Mach-Zehnder device that is insensitive to temperature gradients and able to withstand forces that would tend to cause inadvertent bending has been proposed. The device includes an elongated body of matrix glass through which first and second dissimilar optical fibers extend. The body includes a phase shift region in which the fibers have different propagation constants, whereby optical signals propagate through the optical fibers at different velocities in the phase shift region. At opposite ends of the phase shift region the body further includes two spaced, tapered coupler regions where the diameter of the body and the diameters of the fibers are smaller than in the phase shift region. Even though the propagation constants of the fibers are different in the phase shift region, the difference in propagation constants of the fundamental modes propagating in those fibers inside the tapered coupling regions is insignificant due to the small size of the cores in the tapered regions wherein coupling occurs.
It has been proposed to use overclad Mach-Zehnder couplers for gain flattening filter applications. The typical sinusoidal dependence of the two-coupler device is useful for filtering the red band or the blue band gain of Erbium-doped fiber amplifiers. Broadband gain filters require a nonsinusoidal filter function. Such broadband functionality using a three coupler-two core overclad lattice structure has been demonstrated. However, it has been discovered that two-fiber overclad Mach-Zehnder narrowband filters tend to be polarization sensitive, because the cores deform in the phase shift region of the device as the tube collapses onto the fibers during manufacture of the device.
A coaxial geometry has been proposed to eliminate the polarization sensitivity. Such devices are formed from an optical fiber that defines two waveguides, a rod waveguide (the central core of the fiber) and a coaxial tubular or ring waveguide. The refractive indices of the central core and the ring waveguides are elevated relative to the refractive index of the cladding layer that is intermediate the core and ring waveguides and relative to the refractive index of the outer cladding that surrounds the ring waveguide. Implementation of the design is difficult for the following reasons. In order to couple light from the core waveguide to the ring mode in the ring waveguide, it is required that the propagation constants of these modes in the tapered regions be similar. But dissimilar propagation constants are required for good filtration. It is difficult to form a coaxial fiber coupler that meets these requirements. More importantly, in a coaxial device formed of a fiber having a central core waveguide and a ring waveguide, the ring mode may be so tightly bound to the ring waveguide that it is not readily stripped therefrom by the protective coating of the output fiber pigtail. This might necessitate the utilization of a further bath of index matching fluid to prevent light that propagates in the ring waveguide from reaching the output of the device. If the ring mode reaches the splice between the output pigtail and the system fiber, modal noise is generated. In addition, the characteristic of insertion loss with respect to wavelength of Mach-Zehnder devices made from a coaxial fiber having core and ring waveguides was highly non-reproducible.